The present invention relates to particles suitable for use in electrorheological fluids and electrorheological fluids containing such particles.
Electrorheological ("ER") fluids are fluids which can rapidly and reversibly vary their apparent viscosity in the presence of an applied electric field. ER fluids are generally dispersions of finely divided solids in hydrophobic, electrically non-conducting oils. They have the ability to change their flow characteristics, even to the point of becoming solid, when subjected to a sufficiently strong electrical field. When the field is removed, the fluids revert to their normal liquid state. ER fluids may be used in applications in which it is desired to control the transmission of forces by low electric power levels, for example, in clutches, hydraulic valves, shock absorbers, vibrators, or systems used for positioning and holding work pieces in position.
The prior art teaches the use of a variety of fine particles, some with surface coatings of various types. For example, PCT Publication WO/93/07244, published Apr. 15, 1993, discloses electrorheological fluid comprising polyaniline. The polymer can be formed in the presence of solid substrates such as silica, mica, talc, glass, alumina, zeolites, cellulose, organic polymers, etc. In these embodiments, the polymerized aniline generally is deposited on the substrate as a coating which may also penetrate into the open pores in the substrate.
Japanese Publication 5,239,482, Feb. 28, 1992, discloses inorganic or organic particles, coated with a polyaniline, and the polyaniline-coated particles dispersed as a dispersed phase. The effect is that an electro-viscous fluid having large electro-viscous effects is obtained.
One of the goals in development of a practical electrorheological fluid is to provide a fluid which exhibits a combination of good yield stress and shear stress under field (i.e., high ER activity) and low current density. Moreover, the fluid should have a fast response time, that is, be able to respond to a field within a matter of milliseconds. Finally, the fluid should exhibit this desirable combination over a useful, broad temperature range. The materials of the present invention exhibit such a useful combination of properties.